1. Field Of The Invention
The present invention relates to a process for coating the exterior surfaces of crystalline silica polymorphs, e.g., crystalline silica and aluminosilica type zeolite catalysts with amorphous silica to increase the selectivity of the catalysts.
2. Description Of The Prior Art
The benefits of coating silica containing catalysts such as zeolites, e.g., ZSM 5 and crystalline silicas have been recognized. U.S. Pat. No. 4,203,869 issued on May 20, 1980 discloses a process for coating a zeolite type catalyst to increase the selectivity which process deposits an isocrystalline layer of aluminum-free zeolite over the surface of the zeolite resulting in an aluminum-free outer shell having the same crystal structure as the zeolite. U.S. Pat. No. 4,283,306 issued on Aug. 11, 1981 discloses a group of novel crystalline silicas along with the concept of promoting the catalytic activity of the silicas by the incorporation of certain compounds including amorphous silica. The amorphous silica is placed on the catalyst by the use of certain silicone compounds as described in Column 5, lines 12-38 of the patent. The silicones cannot enter the channels because of their size. However, these compounds are quite expensive relative to the silicates of the present invention. Alumina is coated on the surface of crystalline zeolites by a technique disclosed in U.K. Patent Application No. 2079737 filed on Jul. 6, 1981. U.S. Pat. No. 3,753,929, issued on Aug. 21, 1973, discloses a method for preparing an alumina coated zeolite type catalyst by contacting the zeolite with a soluble aluminum sulfate, or aluminate at a pH 3-5.
Catalysts to which the process of the present invention can be applied are disclosed in U.S. Pat. No. 4,073,865 issued on Feb. 14, 1978, U.S. Pat. No. 4,061,724 issued on Dec. 6, 1977, U.S. Pat. No. 4,330,519 issued on May 18, 1982, U.S. Pat. No. 4,104,294 issued on Aug. 1, 1978, U.K. Patent Application No. 2,084,552 A filed on Sept. 9, 1981 as well as the references disclosed in the foregoing references. The process of the present invention is particularly useful in coating catalysts for the methylation of toluene as disclosed in U.S. Pat. No. 3,965,208 issued on Jun. 22, 1976 and U.S. Pat. No. 4,100,215 issued on Jul. 11, 1978. U.S. Pat. No. 4,127,616 issued on Nov. 28, 1978 which discloses numerous references to disproportionation and alkylation using catalysts which can be coated according to the process of the present invention also discloses coating using bulky silicones.